[clang] Speedup LineOffsetMapping::get
LineOffsetMapping::get is a critical function that consistently appears
in the top 5 more computation intensive functions when running the
preprocessor.
This change brings consistent speedup of ~.5% on, preprocessing time,
see
https://llvm-compile-time-tracker.com/compare.php?from=
0745b0c0354a0c8e1fefb68a3876d15db6c2e27a&to=
460f3f04dac025e6952d78fce104a88151508a29&stat=instructions:u
for detailed statistics.
Differential Revision: https://reviews.llvm.org/D138474